Ventilation of the Cariaco Trench, a case of multiple source competition?

Abstract

The joint evolution of potential temperature, salinity and tritium concentrations in the deep basin in the Cariaco Trench, over a 13 year period from late 1972 to early 1986, suggests a complex pattern of ventilation of the basin. Scranton et al. (1987, Deep-Sea Research, 34, 945–963) found that a purely diffusive model could accommodate the observed evolution from 1973 to 1982 of concentrations of a range of hydrographic and chemical trace species, but the tritium data demand inclusion of two distinct convective ventilation sources with differently distributed depth penetration. Exploratory computations with an extension of Scranton's model, including penetrative convection effects, suggest that a single ventilation mode cannot explain the total observed evolution. The observed trends in temperature, salinity and tritium concentration can be individually matched by fitting simpler models, but jointly they demand injection of warm hypersalline shelf waters which reach the basin bottom, as well as input of Caribbean thermocline waters at the sill. While the overflow events at the sill appear not to have penetrated into the deepest basin layers in recent times, the observed bottom temperature demands at least episodic ventilation with Caribbean thermocline waters colder than 16.5°C. The apparent absence of such activity, for several decades through the mid-1980s, implies climatically persistent differences in the circulation dynamics of the southern Caribbean Sea. A theoretical discussion suggests that this may be but one example of extreme sensitivity of bottom ventilation processes in enclosed deep basins to forcing fluctuations, and that even weak geothermal heating may exert a significant influence on the stratification and ventilation statistics in such systems.